The present invention is broadly concerned with a spinal fusion cage system. More particularly, it is directed to an articulated implant which can be installed between a pair of adjacent vertebrae and selectively expanded in situ to form a wedge with an adjustable angle of inclination for supporting and stabilizing the vertebrae in normal curved alignment in order to promote fusion of the aligned vertebrae.
The spine is a column of stacked vertebrae which are normally axially aligned along the median plane. When viewed from the front or back, the spine appears to be straight. When viewed from a lateral perspective, however, it is shown to be comprised of four distinct curves. Each vertebra is angularly displaced in the coronal plane in accordance with its position along one of the respective curves.
The structure of each vertebra includes a rounded, weight bearing anterior element, or vertebral body, which is separated from the adjacent superior and inferior vertebral bodies and cushioned by fibrocartilage pads or discs. These intervertebral discs support the adjacent vertebrae in an appropriate angular orientation within a respective spinal curve and impart flexibility to the spine so that it can flex and bend yet return to its original compound curvate configuration.
Aging, injury or disease may cause damage to the discs or to the vertebrae themselves. When this occurs, it may be necessary to surgically remove a disc and fuse the adjacent vertebral bodies into a single unit. Such surgical arthrodesis is generally accomplished by implanting a cage-like device in the intervertebral or disc space. The cages are apertured, and include a hollow interior chamber which is packed with live bone chips, one or more gene therapy products, such as bone morphogenic protein, cells that have undergone transduction to produce such a protein, or other suitable bone substitute material. Following implantation, bone from the adjacent vertebrae above and below the cage eventually grows through the apertures, fusing with the bone of the adjacent vertebral bodies and fixing the adjacent vertebrae as well as the cage in position.
Once the disc has been removed from the intervertebral space, the angular orientation of the adjacent vertebrae is established and stabilized by the three dimensional geometry of the implanted fusion cage, and the vertebrae will eventually fuse in this orientation. The lumbar curve presents a region of normal anterior convexity and posterior concavity or lordosis. There is a need for an anterior implant for use in this region which can be adjusted in situ to achieve and maintain normal lordosis of the vertebrae.
Previous attempts to achieve normal spinal curvature with fusion cages have involved trial insertion of cages of various different sizes into the intervertebral space. The cage is repeatedly removed and replaced with another unit of a slightly different size until an optimal angular incline is achieved. There is a need for a modular and articulated implant which can be installed in a first configuration, and adjusted in situ into a wedge configuration from an anterior access position.
Once installed in an intervertebral space, spinal implants are subject to compressive forces exerted by gravity and movement of the spinal column. Normal forward bending activity exerts substantially greater compressive force on the vertebrae than backward bending. Consequently, there is a need for an implant which will accept an increased anterior preload to withstand anterior compressive forces and to maintain the disc space height.
Spinal implants are also subject to twisting forces caused by unequal lateral distribution of weight on the adjacent vertebral bodies. This may occur, for example, during normal sideward bending and reaching activity. There is also a need for an implant which will provide torsional stability to resist such twisting forces. In particular, in order to withstand the greater compressive forces associated with forward bending movements, there is a need for an implant that will provide enhanced anterior torsional stability.
The apparatus of the present invention is specifically designed to provide a modular intervertebral implant which can be both installed and selectively expanded in situ from an anterior access position to form a wedge which stabilizes the adjacent vertebrae in normal curved alignment while providing lateral stability, increased anterior preload and enhanced anterior torsional stability.
The present invention is directed to an articulated modular cage system for implantation in the intervertebral space and adjustment in situ from an anterior access position to support the adjacent vertebrae in a normal curved alignment while permitting fusion of the adjacent bones. The fusion cage system of the present invention includes a first leg having a pivot member, a second leg having a socket for receiving the pivot member and a driver. The socket permits movement of the first leg about an axis of pivotal rotation from a closed, parallel insertion position to an anteriorly open, wedge-shaped orientation which may be selectively adjusted to provide appropriate angular support. The socket and pivot member are laterally elongated to provide lateral support. The pivot member includes a cylindrical notch or aperture, and the socket includes a threaded bore which are aligned for receiving a driver.
The driver is operable to engage a sloped interior surface of the first leg and to urge the anterior end of the first leg apart from the anterior end of the second leg while causing the pivot member to rotate within the socket. Registry of the driver within both the bore and the aperture serves to prevent lateral displacement of the pivot member within the socket. The pivot member and socket are inset or positioned anteriorly of the posterior ends of the respective legs in order to enhance torsional stability and to optimize the anterior preload. This is achieved by decreasing a moment arm length between an effective area of engagement of the adjacent vertebrae and the location of the connection between the legs of the cage. Positioning the pivot axis anterior of the posterior ends of the legs also helps to optimize the intervertebral spacing and angular alignment of the adjacent vertebrae to avoid undesirably stressing the next vertebrae beyond the vertebrae engaged by the fusion cage.